1. Field of the Invention
The present invention relates to reflection sheets for a backlight unit which reduce loss of rays of light, and backlight units in which this reflection sheet is used.
2. Description of the Related Art
Liquid crystal equipments in widespread use have been in a backlight system where light emission is executed by irradiating onto a liquid crystal layer from the back face. In such a type of equipment, a backlight unit which is an edge light type, an immediate beneath type or the like is provided to an under face side of the liquid crystal layer. Such a backlight unit 40 of an edge light type is generally equipped with a rod-shaped lamp 41 for use as a light source, an optical waveguide plate 43 having a square plate shape disposed so that the edge thereof abuts along the lamp 41, multiple optical sheets 42 overlaid to the front face side of the optical waveguide plate 43, and a reflection sheet 46 overlaid to the back face side of the waveguide plate 43, as shown in FIG. 5. Each of the optical sheets 42 has specified optical properties such as refraction, diffusion and the like. Specifically, included are a light diffusion sheet 44 provided on the front face side of the optical waveguide plate 43, a prism sheet 45 provided on the front face side of the light diffusion sheet 44, and the like.
Referring to functions of the backlight unit 40, rays of incident light from the lamp 41 to the optical waveguide plate 43 are first reflected on reflection dots (not shown in the Figure) of the back face of the waveguide plate 43 and on each of the side faces, and exit from the front face of the waveguide plate 43. The rays of light that exited from the waveguide plate 43 enter into the light diffusion sheet 44, then diffuse and exit from the front face of the light diffusion sheet 44. Thereafter, the rays of light exited from the light diffusion sheet 44 enter into a prism sheet 45, and exit as rays having a distribution representing a peak in a direction along a substantially normal line through a prism part 45a formed on the front face of the prism sheet 45. Accordingly, the rays of light exited from the lamp 41 are diffused by the light diffusion sheet 44, while being refracted by the prism sheet 45 so that they represent a peak in a direction along the substantially normal line, and illuminate the entire face of the upper liquid crystal layer although not shown in the Figure.
Meanwhile, a reflection sheet 46 is provided to the back face side of the optical waveguide plate 43 for the purpose of: attempting the improvement of brightness through reflecting rays of light that are radiated from the back face of the optical waveguide plate 43 to the front face side in an extent that glare does not result thereby (reflection property); and preventing instruments, frames and the like that are provided to the back face side of the backlight unit 40 from becoming visible (concealing property).
Examples of such a reflection sheet 46 which may be conventionally used include: (a) sheets made of a synthetic resin including a white pigment such as titanium oxide dispersed therein; (b) sheets made of a synthetic resin including voids (bubbles) dispersed therein for light scattering; (c) sheets having a surface formed into a mat state; and the like.
Because the conventional reflection sheets 46 as described above are comparatively hard, back face of the optical waveguide plate 43 overlaid to the front face side in the backlight unit 40 may get scratched. Particularly, in instances of the optical waveguide plate 43 having a prism part with a gentle tilt to the back face and imparting directional characteristics toward the normal line to rays of light that exited; or comparatively flexible optical waveguide plate 43 made of amorphous olefin resin (for example, “Zeonor optical waveguide plate” manufactured by Nihon Zeon Corporation), there is relatively great possibility to cause scratches onto the back face of the reflection sheet 46. Scratching on a back face of the optical waveguide plate 43 in this manner results in decrease in the amount of light exited to the front face side, and lack in uniformity of the brightness.